Christian's quiet in bed (4)
I believe the answer is:
copt
'christian's' is the definition.
(member of the Coptic Church, a Christian church)
'quiet in bed' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'in' is an insertion indicator.
'bed' becomes 'cot' (cot is a type of bed).
'p' going inside 'cot' is 'COPT'.
(Other definitions for copt that I've seen before include "Member of ancient Egyptian church" , "Early Christian" , "Christian in mainly Moslem country" , "Egyptian Christian Church member" .)